Recently, as turbo compressors used, for instance, when compressed air is produced and supplied to demanding places such as a plant, two-stage turbo compressors and three-stage turbo compressors have been known in response to required pressure of the compressed air. These types of turbo compressors have a plurality of compressor blades rotated by a pinion shaft connected to a large gear shaft via an acceleration device. In the turbo compressors, operations of causing a fluid compressed by first stage compressor blades to be cooled by a cooler, then guiding the cooled fluid to second stage compressor blades to compress the cooled fluid again, and guiding the compressed fluid to a separate cooler to cool the compressed fluid are sequentially performed. Furthermore, an operation of feeding oil to the large gear shaft, the acceleration device, and the pinion shaft of the turbo compressor to lubricate the large gear shaft, the acceleration device, and the pinion shaft is performed, and the oil after the lubrication is collected and circulated in an oil tank.
As the two-stage turbo compressor, a configuration in which the oil tank is integrally assembled to a side portion of a box body housing the cooler is known (see Patent Document 1). However, it is difficult to manufacture the configuration adapted to integrally assemble the box body and the oil tank like the turbo compressor disclosed in Patent Document 1. Accordingly, the two-stage turbo compressor is unfavorable in terms of the productivity and production costs thereof.
As the three-stage turbo compressor, a configuration in which an acceleration unit cover housing the acceleration device, a plurality of compression unit covers housing the compressor blades, and cooler chambers that individually house elongate multi-stage coolers arranged in parallel at a lower portion thereof and are spatially connected between the compression unit covers by fluid passages are formed by a cast integral casing is known (see Patent Document 2).